What are the heaviest objects in the universe?

These interstellar monsters weigh almost too much to comprehend.

Supermassive black holes have masses that are millions and sometimes billions of times greater than our sun.
(Photo: NASA/JPL-Caltech/Wikimedia Commons)

The universe is a big place — really big — and it’s filled with some wondrously weighty objects. The heaviest of them all are black holes and neutron stars. In fact, they weigh so much that it’s nearly impossible to wrap your head around numbers that far off the scale. Here’s a closer look at these mighty mysteries.

Black holes

When matter is packed into an infinitely dense space, the gravitational pull can be so powerful that nothing escapes, including light. That’s a black hole. Scientists can’t see them, but they can observe their gargantuan impact on nearby objects and matter. Their conclusion? Black holes are one of the heaviest things in the universe.

There are many types of black holes. Most common are stellar-mass black holes, which boast a mass three to 20 times that of our sun. That’s big, but the real heavy-hitters are their supermassive counterparts. These behemoths can be billions of times more massive than our sun.

For perspective, the sun weighs about 333,000 times as much as the Earth (which itself weighs an estimated 13 billion trillion tons). Looked at another way, about 1.3 million Earths could fit inside the sun.

Scientists don’t fully understand how supermassive black holes form, but they believe they inhabit the center of every galaxy, including our own Milky Way. Here are some of the most massive supermassives currently known.

1. Black hole in galaxy NGC 4889. This unnamed intergalactic goliath is the current heavy-weight champion. Located in the constellation Coma Berenices about 300 million light-years from Earth, it has a mass 21 billion times greater than our sun. By comparison, the supermassive black hole at the center of our Milky Way galaxy – Sagittarius A* – is only 3 to 4 million times more massive than the sun.

2. Black hole in the quasar OJ 287. This supermassive colossus lurks some 3.5 billion light-years away and weighs in at 18 billion suns. It’s part of a quasar, a highly luminous star-like object consisting of a supermassive black hole surrounded by an accretion disk of spiraling matter and gas. As this material is sucked into the black hole, it heats up, resulting in bright jets of radiation.

What makes OJ 287 so interesting is its unusual light outbursts, which occur roughly every 12 years. The most recent one occurred in December 2015. Researchers now believe the quasar’s supermassive black hole is actually part of a binary system with a second smaller supermassive black hole orbiting it. Every 12 years the more diminutive partner (estimated to have a mass equal to 100 million suns) comes close enough to pop through the larger black hole’s accretion disk and spark the light flare.

3. Black hole in galaxy NGC 1277. Some 250 million light-years away in the constellation Perseus dwells another celestial monster estimated to be 17 billion times more massive than our sun. Bizarrely, this supermassive black hole accounts for about 14 percent of its galaxy’s mass – a far higher ratio than seen in more typical galaxies. Researchers believe NGC 1277 may represent a new type of black hole-galaxy system.

No doubt even heftier supermassive black holes will eventually be discovered. One area ripe for exploration is within the universe’s largest and most radiant galaxy clusters. Scientists have already turned up several in these areas with masses equal to 10 billion suns.

Neutron stars

Stars that are significantly more massive than our (average-sized) sun end their lives in a supernova explosion. Depending on how big they are, one of two things happens. The largest of these stars implode from their own tremendous gravitational force and become stellar mass black holes. Smaller stars that aren’t quite massive enough to collapse into black holes end up compressing into ridiculously dense neutron stars.

Cross section of a neutron star, one of the heaviest objects in the universe. (Photo: NASA/Wikimedia Commons)

These ultra-compact supernova remnants measure only 6 to 12 miles in diameter (about the size of a small city) but have the mass of 1.5 suns. That makes them one of the weightiest objects in the universe. As Andrew Melatos, a professor at the University of Melbourne’s School of Physics, notes: "A teaspoon of neutron star would weigh around a billion tons." That’s equivalent to the weight of 3,000 Empire State Buildings.

Here are the heaviest of the heavies:

1. PSR J1614-2230. Located 3,000 light-years away, this jumbo-sized neutron star has a mass of two suns packed into a space the size of center-city London. PSR J1614-2230 is a pulsar, a rapidly spinning neutron star that emits beams of electromagnetic radiation that sweep around the sky like a lighthouse beacon. This one spins about 317 times a second. Many neutron stars are believed to start out as pulsars but eventually slow down and stop emitting radio waves. PSR J164-2230 has an orbiting companion, a white dwarf star formed after the collapse of a low-mass star less than 10 times the mass of our sun.

2. PSR J0348+0432. Just 12 miles across, this similar neutron star is also a pulsar with the mass of two suns and has an orbiting white dwarf companion.

Scientists recently trained their eyes on a collision of two neutron stars located 130 million light-years away in the galaxy NGC 4993. The smash-up, dubbed a kilonova, was observed in August 2017 and may have resulted in a hyper-massive neutron star (perhaps the biggest one ever observed) or a black hole.